/usr/lib/os-probes/50mounted-tests is in os-prober 1.65+deb8u1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 | #!/bin/sh
# Sub-tests that require a mounted partition.
set -e
partition="$1"
. /usr/share/os-prober/common.sh
types="$(fs_type "$partition")" || types=NOT-DETECTED
if [ "$types" = NOT-DETECTED ]; then
	debug "$1 type not recognised; skipping"
	exit 0
elif [ "$types" = swap ]; then
	debug "$1 is a swap partition; skipping"
	exit 0
elif [ "$types" = crypto_LUKS ]; then
	debug "$1 is a LUKS partition; skipping"
	exit 0
elif [ "$types" = ntfs ]; then
	if type ntfs-3g >/dev/null 2>&1; then
		types='ntfs-3g ntfs'
	fi
elif [ -z "$types" ]; then
	if type cryptsetup >/dev/null 2>&1 && \
	   cryptsetup luksDump "$partition" >/dev/null 2>&1; then
		debug "$1 is a LUKS partition; skipping"
		exit 0
	fi
	for type in $(grep -v nodev /proc/filesystems); do
		# hfsplus filesystems are mountable as hfs. Try hfs last so
		# that we can tell the difference.
		if [ "$type" = hfs ]; then
			delaytypes="${delaytypes:+$delaytypes }$type"
		elif [ "$type" = fuseblk ]; then
			if type ntfs-3g >/dev/null 2>&1; then
				types="${types:+$types }ntfs-3g"
			fi
		else
			types="${types:+$types }$type"
		fi
	done
fi
tmpmnt=/var/lib/os-prober/mount
if [ ! -d "$tmpmnt" ]; then
	mkdir "$tmpmnt"
fi
mounted=
if type grub-mount >/dev/null 2>&1 && \
   type grub-probe >/dev/null 2>&1 && \
   grub-mount "$partition" "$tmpmnt" 2>/dev/null; then
	mounted=1
	type="$(grub-probe -d "$partition" -t fs)" || true
	if [ "$type" ]; then
		debug "mounted using GRUB $type filesystem driver"
	else
		debug "mounted using GRUB, but unknown filesystem?"
		type=fuseblk
	fi
else
	ro_partition "$partition"
	for type in $types $delaytypes; do
		if mount -o ro -t "$type" "$partition" "$tmpmnt" 2>/dev/null; then
			debug "mounted as $type filesystem"
			mounted=1
			break
		fi
	done
fi
if [ "$mounted" ]; then
	for test in /usr/lib/os-probes/mounted/*; do
		debug "running subtest $test"
		if [ -f "$test" ] && [ -x "$test" ]; then
			if "$test" "$partition" "$tmpmnt" "$type"; then
				debug "os found by subtest $test"
				if ! umount "$tmpmnt"; then
					warn "failed to umount $tmpmnt"
				fi
				rmdir "$tmpmnt" || true
				exit 0
			fi
		fi
	done
	if ! umount "$tmpmnt"; then
		warn "failed to umount $tmpmnt"
	fi
fi
rmdir "$tmpmnt" || true
# No tests found anything.
exit 1
 |